1. Field of the Invention
The present invention relates to an image fixing apparatus for use in an image forming apparatus such as a copier machine, a printer, a facsimile machine and a hybrid machine which combines two or more of these three.
2. Description of the Related Art
In an image forming apparatus of an electrophotographic method such as a copier machine and a printer, a surface of a photosensitive member which is an image carrier is electrified, an image is exposed in the electrified area to thereby form an electrostatic latent image, the electrostatic latent image is developed into a visualized toner image, the toner image is transferred onto a recording medium such as a recording paper, and an image fixing apparatus fixes the toner image. Meanwhile, in an image forming apparatus of a direct recording method, a toner is blown in accordance with a received image signal onto a recording medium such as a recording paper from a line of toner supply holes which are formed in a hopper which contains the toner to thereby directly form a visualized toner image without forming an electrostatic latent image, and an image fixing apparatus fixes the visualized toner image. An image fixing apparatus as such usually comprises a fixing roller which contacts an unfixed toner image on a recording medium and a pressure roller which is urged against the fixing roller under pressure and accordingly rotates. The unfixed toner image is subjected to heat and a pressure together with the recording medium while transporting the recording medium carrying the toner image in a nip portion between the two rollers, whereby the toner image is fixed on the recording medium.
Known as a method of heating the fixing roller are a method which requires to heat up the fixing roller from inside by a heater which is disposed inside the fixing roller and a method which requires to heat up the fixing roller from outside by means of a heater which is disposed outside the fixing roller. As compared with the method requiring to heat up the fixing roller from inside, the method requiring to heat up the fixing roller from outside is characterized in that it is possible upon startup of a fixing apparatus to shorten a time which is necessary to increase a temperature of the surface of the fixing roller to an appropriate temperature for fixing and in that when a large number of record images are to be outputted continuously, it is possible to quickly increase a temperature of the surface of the fixing roller which tends to decrease due to heat stolen by the recording medium which moves passing the fixing apparatus. Another characteristic is that it is possible to ensure a wider nip width, as a surface layer of the fixing roller is thickened.
However, the method heating up the fixing roller from outside demands to preliminarily operate the fixing apparatus before a paper arrives, for the purpose of increasing a temperature of the fixing roller to a temperature which is needed for fixing. Hence, when a material whose heat transfer coefficient is large is used for the surface layer of the fixing roller, heat flows into a roller base and escapes, and therefore, a long time is needed until a temperature of the surface of the fixing roller increases to a temperature which enables fixing since an image formation start signal is sent. On the other hand, when a material, such as an expanded rubber, whose thermal conductivity is low is used for the surface layer in an effort to decrease the heat transfer coefficient and accordingly quickly increase a temperature of the fixing roller, a temperature of the surface layer of the fixing roller decreases at a rear end of a recording paper which travels through the fixing apparatus, thereby causing a large difference in gloss of an image within the same recording paper, defective fixing on the rear end side, etc.